Device for feeding reactor initiators

ABSTRACT

In a process for preparing polyethylene in tube reactors with or without autoclaves, where a free-radical initiator is introduced with or without cold ethylene into a flowing ethylene- and comonomer-containing medium, rotation is generated between two streams ( 61, 62 ) to be mixed at an angle ( 66 ) or by provision of a swirl element ( 20, 80 ) in the flow cross section ( 27, 28 ). In the region of a feed point ( 72, 81 ) for a free-radical initiator, there is provided a cross-sectional constriction ( 63, 67, 71 ) at which the free-radical initiator is introduced through an optimized off-center outlet opening ( 44 ) of an introduction finger ( 40 ) into the rotating flow ( 61, 62, 70 ).

[0001] The present invention relates to an apparatus for feedinginitiator into reactors, for instance feeding peroxide intohigh-pressure reactors for producing LDPE.

[0002] Polyethylene (PE) is one of the most important plastics and has ahigh resistance to aqueous acids and alkalis. The plastic has goodelectrical properties such as a low dielectric constant and a highspecific resistance. Furthermore, this plastic combines good mechanicalproperties such as a high impact toughness with low densities, whichmakes it suitable for use in many technical fields. Thus, films andconsumer articles for domestic and industrial use are produced from PE;polyethylene is also employed for cable insulation and pipe sheathing.Low density polyethylene (LDPE) has a high transparency because of itslow crystalline content of only 50-70% compared to high densitypolyethylene (HDPE) in which the crystalline content is 70-90%, and thisfavors its use as a film material. A widely used method of producingpolyethylene films is calendering, by means of which polyethylene filmshaving thicknesses in the range from 0.05 to 1 mm can be produced. Incalendering, the thermoplastic is rolled out between many rolls betweenwhich the thermoplastic is molded to form an ever thinner film. Afterleaving the calender, the film is cooled on cooling rolls andsubsequently rolled up.

[0003] One process for preparing LDPE is the tube reactor process. Atthe beginning of the polymerization, peroxide initiators are introducedin liquid form into the tube reactor. Compared to the amount ofethylene, the mass flow of the peroxide initiator is only small. Aproperty of the initiator used is that it quickly decomposes into freeradicals under the conditions prevailing in the tube reactor. To achievea high effectiveness of these initiators, for example peroxide, so as toensure a high conversion, improved polymer properties and more stablereactor operation, it is advantageous to mix the reactants with oneanother very quickly.

[0004] EP 0 980 967 discloses a process for preparing ethylenehomopolymers and copolymers in a tube reactor at pressures above 1000bar and temperatures in the range from 120° C. to 350° C. byfree-radical polymerization. Small amounts of free-radical initiatorsare firstly introduced into a flowing medium comprising ethylene, molarmass regulators and optionally polyethylene, after which polymerizationoccurs. According to this process, the flowing medium is firstly dividedinto two volume elements flowing separately from one another and theseparately flowing volume elements are then set into relativecontrarotation by means of suitable flow-directing elements. Thecontrarotating, flowing volume elements are subsequently recombined toform a flowing medium and at the time of or shortly after thecombination of the contrarotating, flowing volume elements, thefree-radical initiator is introduced into the sheared boundary regionbetween the contrarotating flowing volume elements. EP 0 980 967 alsodiscloses an apparatus for carrying out this process. An improvement inmixing of the initiator metered in and, associated therewith, animprovement in the product quality was also able to be achieved byincreasing the flow velocity in the mixing zones.

[0005] The effectiveness of the free-radical initiator chosen depends onthe rapidity with which it can be mixed with the reaction mediuminitially present in an individual case. For this purpose, injectionfingers are used in industrial plants for the production ofpolyethylene.

[0006] EP 0 449 092 A1 describes the introduction of free-radicalinitiators, initiator mixtures or solutions of initiators in organicsolvents via injection fingers at a plurality of points along a reactor.

[0007] U.S. Pat. No. 4,135,044 and U.S. Pat. No. 4,175,169 describe howa comparatively small tube diameter in the initiation and reaction zonesof a high-pressure reactor, relative to the enlarged tube diameter inthe cooling zone, makes it possible to produce products having very goodoptical properties in high yields and at a relatively small pressuredrop over the length of the reactor.

[0008] Finally, U.S. Pat. No. 3,405,115 disclosed that uniforminitiation of the polymerization reaction and optimum mixing of thereaction components are of great importance for the quality of thepolyethylene obtained, for high reactor yields and for achieving uniformreactor operation. According to this solution, initiators are mixed withsubstreams of cold ethylene in a special mixing chamber and only thenare introduced into the actual reactor. In the mixing chamber, the fluidin which the initiator does not decompose because of the lowtemperatures prevailing there is multiply diverted and passed throughchannels.

[0009] It is an object of the present invention to further optimize theintroduction of a free-radical initiator into a flowing medium so as togive as high a mixing speed as possible.

[0010] We have found that this object is achieved by a process forpreparing polyethylene in tube reactors and/or in combination withautoclaves, in which a free-radical initiator is introduced into aflowing ethylene- and possibly comonomer-containing medium and whichcomprises at least the following steps:

[0011] generation of rotation by mixing two streams to be mixed at anangle or generation of rotation in the flowing medium by means of swirlelements,

[0012] provision of a cross-sectional constriction with an inlet zoneupstream of the feed point for a free-radical initiator into a reactiontube,

[0013] introduction of the free-radical initiator into the rotating flowof the flowing medium and

[0014] provision of a downstream mixing zone and a cross-sectionalwidening with an outlet.

[0015] The particular advantage of the method according to the presentinvention is that a more sparing introduction of free-radical initiatorcan be achieved by increasing the effectiveness of mixing. Thegeneration of rotation in the flowing medium increases the turbulencewhich results per se in an improvement in the effectiveness of mixing bymeans of transverse impulse exchange in the fluids to be mixed. Theprocess according to the present invention makes it possible to preparepolyethylenes which can be used to produce films having significantlyimproved optical properties, specifically in respect of transparency,because of lower proportions of high molecular weight material. Thesolution provided according to the present invention and the rapidmixing of the polyethylene-containing flowing medium with thefree-radical initiator enables significantly more stable reactoroperation at extraordinarily high maximum temperatures to be achievedwithout the final product tending to decompose. Furthermore, a fastertemperature rise in the reactor and a better low-temperature initiationbehavior of the polymerization when using initiators which decompose atlow temperature can be achieved. A further advantage of the processaccording to the present invention is the extremely short mixing-in timecompared to the half-life of the initiator.

[0016] In a further embodiment of the idea underlying the invention, thefeed point for the free-radical initiator is located downstream of thepoint at which rotation is generated in the flowing medium. This ensuresthat the free-radical initiator fed into the flowing medium at the feedpoint always enters a flowing medium which is already in a turbulentstate, so that the mixing time is reduced and the effectiveness ofmixing is significantly improved.

[0017] The geometry of the feed orifice of the element for feeding thefree-radical initiator into the rotating, flowing medium makes itpossible to influence the depth to which the free-radical initiator isinjected into the flowing medium. If the introduction orifice for thefree-radical initiator on the injection finger is made particularlysmall, a fine jet of the free-radical initiator can be injected verydeep, relative to the tube cross section, into the flowing medium.Depending on the flow velocity of the flowing medium, the injectiondepth of the free-radical initiator and thus the achievableeffectiveness of mixing can be positively influenced and matched bymeans of the geometry chosen for the feed orifice.

[0018] In one embodiment of the process according to the presentinvention, the feed devices for substreams of the flowing medium are atan angle of 90° to one another. This enables a tangential flow componentto be generated in the resulting stream of the flowing medium and thisflow component generates circumferential rotation in the combined streamof the flowing medium, which is desirable for achievement of turbulentflow. Before the substreams of the flowing medium are combined at anangle of 90° to one another, they can each pass through cross-sectionalconstrictions so that the flow velocity can, depending on the ratio ofthe free to constricted flow cross section, be doubled. If thesubstreams of the rotating flowing medium are combined within thereaction tube, a further increase in the turbulence of the combinedflowing medium can be achieved by provision of a further cross-sectionalconstriction upstream of the feed point for the free-radical initiatorafter passage through an annular space.

[0019] The introduction of the free-radical initiator at the feed pointis preferably into a shear gap of the rotating flowing medium whichrotates in the circumferential direction in the flow cross sectionrelative to the position of the feed point for the free-radicalinitiator.

[0020] Another variant of the generation of a rotating flow comprisesproviding swirl elements in the free flow cross section over which theflowing medium passes and by which the flowing medium is set intorotation in the circumferential direction in the flow cross section, sothat shear gaps arise.

[0021] Rotation in the flowing medium can be generated, on the one hand,in such a way that a core stream is surrounded on its imaginarycylindrical outer surface, i.e. the shear surface, by an annular streamwhich has been set into rotation relative to the core stream. Theannular stream surrounding the core stream can rotate either clockwiseor anticlockwise around the core stream. On the other hand, it is alsopossible to make the core stream rotate and to generate rotationopposite to the rotation of the core stream in a stream surrounding thecore stream.

[0022] The object of the present invention is also achieved by anapparatus for preparing polyethylene in tube reactors, in which afree-radical initiator is fed into a flowing ethylene- and possiblycomonomer-containing medium and the flowing medium is conveyed through areaction tube having a changing flow cross section and a free-radicalinitiator is introduced in a mixing region of the reaction tube andeither substreams of the flowing medium impinge on one another at aparticular angle or swirl-generating elements are located in the flowcross section, with a feed element having an off-center inlet orificefor a free-radical initiator being located downstream of a constrictionin the rotating flow.

[0023] The apparatus according to the present invention for preparingpolyethylene is given tremendous mixing effectiveness by thefree-radical initiator being fed into shear gaps of a rotating flow,which have not only an axial flow component but also flow components inthe circumferential direction. Flow components in the circumferentialdirection effect impulse exchange transverse to the flow direction andthus provide the basis for effective mixing of a plurality of materials.

[0024] In a preferred embodiment of the apparatus of the presentinvention, the outlet orifice at the tip of the feed element, which isconfigured as a flow-favorable injection finger is preferably inclinedat 45° to the axis of the finger. Depending on the cross-sectionaldiameter of the orifice, any angles in the range from 0° to 90° arepossible. The swirl elements which are located in the free flow crosssection in the reaction tube have, on their outer circumference, swirlblades which extend over an annular space of the reaction tube by ineach case about 90° in the circumferential direction. In an alternativeembodiment of a swirl element, the swirl blades are arranged on itsouter circumference so that they extend over an annular space of thereaction tube by in each case about 120° in the circumferentialdirection.

[0025] A further improvement in the mixing effectiveness can be achievedby the flow diameter in the region of the feed point for free-radicalinitiator being reduced to about 70% of the free flow diameter. Thisenables the flow velocity to be increased by a factor of 2, whichlikewise makes a great contribution to the effectiveness of mixing.

[0026] To avoid “deadwater” regions, the transition from the free flowcross section upstream of the constriction to the latter forms a totalangle of from 20° to 40°, so that an abrupt transition is avoided. Thetotal angle is particularly preferably 30°. To improve the mixingbehavior, the diameter of the constriction downstream of the feed pointfor the free-radical initiator is maintained over a mixing sectionlength of from about 10 to 20 tube diameters (D). After this mixingsection of from 10 to 20 tube diameters (D), the mixing section thenwidens at a total angle of less than 20° back to the free flow crosssection. To prevent demixing phenomena in the transition from thenarrower flow cross section to the wider flow cross section as a resultof the decrease in the velocity, the total angle is preferably less than14°, so that a gradual transition from the mixing section cross sectionof 0.7×D to D occurs.

[0027] The invention is described in more detail below with the aid ofthe drawing.

[0028] In the drawing,

[0029]FIG. 1 shows an in-principle sketch of a mixing section withmixing region and injection point for a free-radical initiator

[0030]FIG. 2 shows a swirl-generating component,

[0031]FIG. 3 shows a casing of the swirl element,

[0032]FIGS. 4 and 4.1 show an exterior swirl element,

[0033]FIGS. 5 and 5.1 show an interior swirl element,

[0034]FIGS. 6 and 6.1 show a flow-favorable injection finger,

[0035]FIG. 7 shows an injection point for a free-radical initiatorlocated downstream of a swirl generator and upstream of a mixingsection,

[0036]FIG. 8 shows a T-shaped connecting piece,

[0037]FIGS. 9, 9.1 and 9.2 show swirl-generating internals in flow crosssections with 90° and 120° blade configurations upstream of theinjection of a free-radical initiator.

[0038]FIG. 1 depicts an in-principle sketch of a mixing section withmixing region and a feed point for a free-radical initiator.

[0039] The reaction tube 1 depicted in the schematic diagram of FIG. 1can be part of a tube reactor in which polyethylene LDPE is prepared bythe process proposed according to the present invention. The reactiontube 1 has an inlet cross section 2 and an outlet cross section 3. Onthe inlet side, the reaction tube 1 is connected via a line system witha system for supplying reactants. Both a stream comprising fresh gas andthe unreacted monomer recirculated via the high-pressure return circuitare fed into the mixing vessel 4 as fluctuation damper with buffer. Athrottle element 5 can be located upstream of the mixing vessel.Downstream of the mixing vessel 4, the reactant feed line is providedwith a compressor 6 by means of which the reactants, i.e. the flowingmedium going to the reaction tube 1, are compressed.

[0040] In the feed region 11, a free-radical initiator is fed via afree-radical initiator inlet line 7 into the interior of the reactiontube 1. For this purpose, a feed line system 7 via which a stock 8 of afree-radical initiator is supplied via a throttle element 9 and via acompressor 10 located downstream thereof to the feed point at which thefree-radical initiator, which initiates the polymerization reaction, isintroduced into the flowing medium in the reaction tube 1 is provided.The feed region 11 is followed in the flow direction 12 by a mixingregion 13 which preferably has a length of from 10× to 20× the diameter(D) of the reaction tube 1. The flowing fluid medium which is mixed inthe manner indicated below with the free-radical initiator introduced inthe feed region 11 passes through the mixing section 14.

[0041] The flow cross section of the reaction tube 1 is denoted byreference labels 16 or D. The outlet end 3 of the reaction tube 1 isadjoined by a pressure maintenance valve 15 by means of which thereaction mixture obtained is depressurized. This results in phaseseparation.

[0042] In industrial plants for preparing LDPE, the pressure maintenancevalve 15 shown in the in-principle sketch of FIG. 1 serves as responsevalve and regulating valve. By means of this valve and a downstreamhigh-pressure separator 19.1, part of the flowing, ethylene-containingmedium is, on an industrial scale, returned after cooling to the plantvia a high-pressure circuit 19.3 and the LDPE obtained is passed to ahigh-pressure separator 19.1 from which the product 19.2 is subsequentlytaken off.

[0043] In industrial plants, the reaction tube 1 of a tube reactor isprovided in the mixing region 13 and in the following mixing section 14with wall cooling 18. The wall cooling 18 is usually configured as acooling jacket which removes part of the heat of reaction evolved in thepolymerization reaction between the flowing medium and the free-radicalinitiator. The remainder of the heat of reaction remains in the flowingmedium. In addition, when the process of the present invention isemployed on an industrial scale, in which case a plurality of reactiontubes 1 each forming a reaction stage may be connected in series, themixing sections 14 can each be provided with cold gas inlet lines 17 a,17 b. Mixing-in a cold gas stream at the beginning of the mixingsections 14 allows a further part of the heat evolved in thepolymerization reaction to be compensated in the flowing mixture offlowing medium and free-radical initiators, which is relevant to theconversion. Furthermore, the free-radical initiator can be introducedinto the cold gas stream 17 b via the pump 10.

[0044]FIG. 2 shows a more detailed view of a swirl-generating componentwhich can, for example, be installed in the reaction tube 1 shownschematically in FIG. 1.

[0045] The swirl element 20 depicted in FIG. 2 is accommodated in anouter tube 22. The outer tube 22 in turn encloses an inner tube 23. Onthe outside of the inner tube 23 there are located, as shownschematically in FIG. 2, swirl-generating exterior blades 25 whose swirlblade area 36 decreases in the direction of the outlet cross section 28of the swirl element 20. 2, 3, 4 or more exterior swirl blades 25 can belocated opposite one another on the outer circumference of the innertube 23. The interior of the inner tube 23 can, as shown in theembodiment in FIG. 2, be provided with an interior swirl blade 26. Thisgives the part of the stream passing through the interior cross sectionof the inner tube 23 a rotational motion for generating turbulent flow,while the part of the fluid medium passing through the annular spacebetween inner tube 23 and outer tube 22 is provided with a flowcomponent in the circumferential direction by means of the 2, 4 or moreexterior blades 25 located on the outer circumference of the inner tube23. At the outlet cross section 28 in the region of the points 34 of theswirl blades there is accordingly a rotating flow having acircumferential component relative to the center line 29.

[0046]FIG. 3 shows a casing of the swirl element depicted schematicallyin FIG. 2.

[0047] The casing of the swirl element 20 consists essentially of theouter tube 22 which is located between two flanges 21. The inlet crosssection 27 is parallel to the outlet cross section 28 of the swirlelement 20 coaxial with the center line 29. The inner wall 30 of theouter tube 22 represents the outer boundary of an annular gap which isformed between the outer surface of the inner tube 23 and the outer tube22 and through which the exterior blades 25 which are fastened to theexternal circumference of the inner tube 23 pass in a screw-likefashion.

[0048]FIGS. 4 and 4.1 show an inner tube 23 provided with exteriorblades located opposite one another on the circumferential surface ingreater detail.

[0049] The exterior blades 25 of which, in the embodiment depicted inFIG. 4, two are fixed opposite one another on the outer wall of theinner tube 23 are attached to the inner tube 23 along a line ofattachment 35. The swirl blades 25 extend along the line of attachment35 on the outer surface of the inner tube 23 in a screw-like fashion,with the screw line chosen here having a high pitch. It is also possiblefor more than the two exterior blades 25 shown in FIG. 4 to be providedon the exterior wall 33 of the inner tube, for example four or even sixblades symmetrically at 90° relative to the center line 29.

[0050]FIG. 4.1 shows a plan view of the rear part of the inner tube 23.In FIG. 4.1, the exterior blades 25 on the exterior wall 33 of the innertube are surrounded by the outer tube 22 of the swirl element 20. Inaddition, an interior swirl blade 26 which extends in a twisting fashionover a region of at least 90° along the inner wall of the inner tube 23is provided in the interior of the inner tube. This region can also beup to 180°. It is also possible for a plurality of flow channels to beformed.

[0051]FIGS. 5 and 5.1 show a side view of an interior swirl blade 26 andalso a rear view thereof. Relative to its center line 29, the interiorswirl blade 26 is provided with a twisted interior swirl blade surface37 which, as can be seen in FIG. 5.1, covers a 90° sector of the innersurface of the inner tube 23.

[0052] The screw-like pitches of the exterior blades 25 and the interiorblades 26 have the same sense; the exterior blades 25 and the interiorblades 26 can be fitted to a swirl element as shown in FIG. 20 withdifferent pitches relative to one another. By means of thisconfiguration, the component of the flowing medium flowing through theinterior of the inner tube 23 can be given a counterclockwise rotationwhile the fluid component flowing between the exterior wall 33 of theinner tube and the inner surface 30 of the outer tube 22, i.e. in theannular space, has a clockwise rotation component imparted to it. It canbe seen from the details in FIG. 5 that all edges of the exterior andinterior swirl blades 25 and 26, respectively, which point in the flowdirection or in the direction opposite to the flow are streamlined toavoid eddy formation.

[0053]FIGS. 6 and 6.1 show a side view and plan view, respectively, ofan introduction element for the free-radical initiators, which ispreferably configured as a flow-favorable injection finger.

[0054] The introduction element is let into the wall of a reaction tube1 and is provided with a cone tip 41. The introduction element 40 has ahole 43 which, via a conical narrowing of the cross section, goes overinto a constricted hole which is adjoined by an outlet orifice 44 at anangle 45. The angle of the outlet orifice 44 is, for example, 45° to theaxis of symmetry of the feed element 40, with an angular range from 0 to180° being possible, so that an oblique introduction of a free-radicalinitiator into a flowing medium can be achieved. The depth to which thefree-radical initiator penetrates into the rotating flowing medium canbe adjusted as a function of the angle and cross-sectional area of theoutlet orifice 44 and the flow of the cold gas stream 17, so that thedepth to which the free-radical initiator, e.g. peroxide, penetratesinto the flowing medium can be set independently of the degree ofturbulence generated. At the cone tip 41 of the finger-shaped feedelement 40, the outlet orifice 44 for the free-radical initiator ispositioned so that its circumference preferably enters a shear gap inthe rotating flowing medium. The parameters turbulence and injectiondepth of the free-radical initiator result in the high effectiveness ofmixing in the process proposed according to the present invention andthe apparatus for the preparation of polyethylene proposed according tothe present invention. The outlet orifice 44 on the cone tip 41 of thefeed element 40 is slightly offset from the center line of the feedelement 40. When injection is carried out without a cold gas stream 17,the angle is preferably from 0 to 15°. When a cold gas stream 17 isemployed, the angle is preferably 45° or can be selected within a rangefrom 30 to 60° to prevent the introduced stream from contacting thewall.

[0055] The flow-favorable injection finger 40 whose outlet orifice 44points in the flow direction of the flowing medium prevents theformation of deadwater regions downstream of it. This advantageouslyprevents regions in which there are relatively high concentrations ofthe free-radical initiator forming as a result of eddies in the flow;such high concentrations would otherwise lead to decomposition reactionswhich have a severe adverse effect on the product quality of the LDPE.

[0056] In place of the introduction of the free-radical initiator viathe injection finger 40, the initiator can also be introduced by meansof a carrier medium. Thus, the free-radical initiator, e.g. peroxide,can be introduced into the flowing medium in the cold gas inlet line 17which would then have to be run, as shown in FIG. 1, into the injectionregion 11 of the reaction tube. In place of cold gas as carrier mediumfor the free-radical initiator, it is also possible to use cold ethylenebranched off immediately downstream of the compression stage 6 ascarrier gas for the free-radical initiator. If the free-radicalinitiator is introduced using cold gas as carrier gas, the cold gas andfree-radical initiator can be mixed in a mixing chamber and thispremixed stream can then be injected into the flowing medium at aconstriction, so that, when the introduction orifices and introductionangles are designed appropriately, a high impulse is achieved at thepoint of introduction.

[0057]FIG. 7 shows an injection point for a free-radical initiator,which is located downstream of a swirl-generating element and upstreamof a mixing section.

[0058] A swirl element 20 with exterior swirl blades 25 is assigned toan orifice 51 which projects into a constricted flow cross section andthrough which a free-radical initiator is introduced into the flowingmedium. The exterior swirl blades 25 are located on the outer tube 22 ofthe swirl element 20 which has a length 87, preferably from about 1 to3×D. The swirl element 20 imparts a rotation to the flowing mediumwhich, after passing through a constricted cross section, enters theinjection region 11 for the free-radical initiator at an acceleratedvelocity.

[0059] In the embodiment shown in FIG. 7, the orifice 51 is at the endof a tube 53 which is surrounded by a lens-shaped body 50 which isaccommodated between two sections of the reaction tube 1. Due to thepressure of the free-radical initiator, it is injected into the flowingmedium without contacting the inner wall 52 in the mixing region 11 ofthe reaction tube. After injection of the free-radical initiator intothe medium flowing in the flow direction 12, 24, the reacting mixtureenters a mixing section 14 which can be followed by a widening of theflow cross section not shown here.

[0060] In place of a feed point for pure free-radical initiator 72, 81,the initiator can, in the embodiment shown in FIG. 7, also be introducedby means of a carrier medium, either cold gas 17 or an ethylene streambranched off upstream of the compression stage 6 (FIG. 1). Thefinger-shaped configuration of the introduction element 40 results inbeing formed no deadwater regions being formed downstream in the mixingregion 11, so that flow regions having a relatively high free-radicalinitiator concentration do not occur.

[0061]FIG. 8 shows a T-shaped connecting piece on a reaction tube inwhich two reactant streams are mixed with one another.

[0062] On the reaction tube shown in FIG. 8, a first substream 61 and asecond substream 62 flow to an introduction point on the reaction tubeat an angle 66. The first substream 61 of the reactant present as aflowing medium passes through a first cross-sectional constriction 63which is configured as a conical constriction 64 on the reaction tube.At an angle of 90° thereto, the second substream 62 of reactants flowsdownward in a vertical direction through a conical section 67 toward thereaction tube. Both substreams of the reactants present as flowing mediaexperience acceleration during passage through the respectivecross-sectional constrictions 63 and 67 before the second reactantstream experiences a deflection 66 of 90° and accordingly generates atangential flow 69. The tangential flow 69 occurs in the circumferentialdirection relative to the flow direction of the first substream 61,within an annular space 68 in the reaction tube 1. The substreams 61, 62of the reactant experience, due to the combination at an angle of 90°,mixing by introduction of a tangential flow component 69 into the fluidflowing along the reaction tube.

[0063] The fluid from the substream 62 in the annular space 68 in thereaction tube flows along the annular space 68 between the inner wall ofthe reaction tube and the outer wall of an insert element 65 and iscombined with substream 61 at the end of the insert element 65. Thecombined stream passes the feed point 72 for the free-radical initiator,e.g. peroxide, and a further cross-sectional constriction 71. Thecross-sectional constriction 71 is preferably configured so that thefree flow cross section at the feed point 72 for the free-radicalinitiator, for example peroxide, is preferably 0.7×D (free tubediameter). As a result, the rotating, accelerated and combined stream 70made up of the substreams 61 and 62 of the reactant is subjected tofurther acceleration. If the feed point 72 for the free-radicalinitiator on the tube wall is configured as a finger-shaped,flow-favorable injection element 40 as shown in FIG. 6 and 6.1, afree-radical initiator is preferably introduced at shear gaps into therotating flow provided with a tangential flow component 69 so that rapidand effective mixing of the combined reactant stream is achieved. Thetotal angle at which the cross-sectional constriction 71 goes over fromthe original flow cross section D to 0.7×D is in the range from 20° to40°, particularly preferably a total angle of 30°.

[0064] The mixing section which follows the feed point 72 for thefree-radical initiator preferably has a length of from 10×D to 20×D(D=tube diameter), but can also be 100×D, before there is, after themixing section, a transition to the original flow diameter D. Thetransition from the mixing section diameter of 0.7×D to D preferablyhas, similar to a diffuser configuration, a total angle of from 10 to20°, particularly preferably a total angle of less than 14°.

[0065] Another embodiment of the apparatus proposed according to thepresent invention for the preparation of polyethylene is shown in FIGS.9.1 and 9.2.

[0066] In these embodiments, the reactant stream 61 is conveyed as asingle stream to a cross-sectional constriction 41 [sic]. A divisioninto substreams 61, 62 entering at inlet points at an angle to oneanother is not provided for in this embodiment.

[0067] The constriction 71 goes over at a total angle of 30° into anarrowed flow cross section in a manner analogous to the embodimentdepicted in FIG. 8. After passage through the constriction 71, the flowcross section in the reaction tube is 0.7×D, which is maintained overthe mixing section which follows the feed point 81 for the free-radicalinitiator. The length of the mixing section is preferably from 10×D to20×D (D=original reaction tube diameter).

[0068] After the constriction 71, at which the flow velocity isincreased by a factor of up to 2, swirl elements 80 are installed in thefree flow cross section of the reaction tube. The swirl elements 80 arelocated, based on the flow direction 24, upstream of the feed point 81for a free-radical initiator such as peroxide. In the embodimentdepicted in FIG. 9.1, two swirl blades 82 are located on the outercircumference of the swirl elements 80. In this configuration, the swirlblades each extend 90° around the external circumferential surface ofthe swirl element 80 s, so that a rotation is imparted to the fluidstream which enters at increased velocity. The ends of the swirl blades82 fitted to the outer surface of the swirl elements 80 touch the insideof the reaction tube 1 which encases the swirl elements 80. The edge 85of the blades 82 on the outer surface 84 of the swirl elements 80 form aseal so that the fluid passing the swirl element 80 is forced throughthe annular space between outer surface 84 and inner wall of thereaction tube, thus ensuring generation of a flow component in thecircumferential direction during passage past the swirl element 80.

[0069] An alternative possible embodiment comprises, as shownschematically in FIG. 9.2, installing a swirl element 80 in the regionof the reaction tube downstream of the constriction 71, with the swirlblades 82 fitted to the outer surface 84 of the swirl body 80 nowextending 120° around the circumferential surface 84 of the swirlelement 80, as indicated by reference numeral 88. In this embodiment ofthe present invention, too, rotation is imparted to the reactant flowinto which a free-radical initiator is to be introduced at theintroduction point 81, as a result of which the mixing conditionsdownstream of the introduction point 31 for the free-radical initiator,e.g. peroxide, are significantly improved. The degree of turbulence canbe influenced firstly by the pitch of the swirl blades 82 and by thelength 87 of the swirl elements. Secondly, the achievable mixingeffectiveness can be optimized by the design of the constriction 71 byacceleration of the reactant stream.

[0070] Significant parameters are, apart from the mixing parameters, thelength of the mixing zone and the acceleration of the flowing medium.

[0071] An aspect common to the embodiments shown in FIG. 8 and FIGS. 9.1and 9.2 is that firstly the generation of rotation can be carried out onintroduction of the substreams 61 and 62 of the reactant, secondly arotating flow can be achieved by angled combination of the substreamsand thirdly rotation can be imparted to the fluid into which afree-radical initiator is to be introduced by means of the swirl element20, 80 located in the flow cross section. The introduction of thefree-radical initiator can be carried out either without or with coldethylene.

[0072] The internals employed according to the present invention forgenerating rotation can also be retrofitted to existing plants afterslight modifications in order to increase their efficiency.

[0073] List of Reference Numerals

[0074]1 Reaction tube

[0075]2 Inlet

[0076]3 Outlet

[0077]4 Mixing vessel

[0078]5 Throttle element

[0079]6 Compressor

[0080]7 Inlet line for free- radical initiator

[0081]8 Initiator reservoir

[0082]9 Throttle element

[0083]10 Compressor

[0084]11 Injection region swirl blade

[0085]12 Flow direction

[0086]13 Mixing region

[0087]14 Mixing section

[0088]15 Valve

[0089]16 Flow cross section

[0090]17 a Inlet line for cold gas

[0091]17 b Inlet line for cold gas

[0092]18 Wall cooling

[0093]19 Fresh gas feed

[0094]19.1 Separator

[0095]19.2 Product

[0096]19.3 High-pressure recirculation

[0097]20 Swirl element

[0098]21 Flange

[0099]22 Outer tube constriction

[0100]23 Inner tube

[0101]24 Flow direction

[0102]25 Exterior swirl blade

[0103]26 Interior swirl blade

[0104]27 Inlet cross section

[0105]28 Outlet cross section

[0106]29 Center line

[0107]30 Interior wall

[0108]31 Exterior wall

[0109]32

[0110]33 Exterior wall of inner tube

[0111]34 Point of swirl blade

[0112]35 Line of attachment

[0113]36

[0114]37 Surface of interior swirl blade

[0115]40 Injection finger

[0116]41 Cone tip

[0117]42

[0118]43 Hole

[0119]44 Outlet orifice

[0120]45 Angle

[0121]50 Injection lens

[0122]51 Orifice

[0123]52 Interior wall

[0124]53 Tube

[0125]60 T-piece

[0126]61 First stream

[0127]62 Second stream

[0128]63 Cross-sectional constriction

[0129]64 Conical section

[0130]65 Insert

[0131]66 90° dimension

[0132]67 Conical section

[0133]68 Annular space

[0134]69 Tangential flow

[0135]70 Constriction 61, 62

[0136]71 Cross-sectional constriction for combined stream

[0137]72 Injection of free-radical initiator

[0138]73 Shear gap

[0139]80 Swirl element

[0140]81 Injection of free-radical initiator

[0141]82 Swirl blade

[0142]83 Extent of swirl blade 90°

[0143]84 Outer surface of swirl element

[0144]85 Edge of blade

[0145]86 Annular space

[0146]87 Length of swirl element

[0147]88 Extent of swirl blade 120°

We claim:
 1. A process for preparing polyethylene in tube reactorsand/or in combination with autoclaves, in which a free-radical initiatoris introduced into a flowing medium comprising ethylene and possiblycomonomers and which comprises at least the following steps: generationof rotation by mixing two streams to be mixed (61, 62) at an angle (66)or generation of rotation in a stream (61) by means of a swirl element(20), provision of a cross-sectional constriction (63, 67; 71) with aninlet zone upstream of the feed point (72, 81) for a free-radicalinitiator into a reaction tube (1), introduction of the free-radicalinitiator through an off-center outlet orifice (44) into the flowing,rotating medium (61, 62; 70) and provision in a downstream direction ofa mixing zone and a cross-sectional widening with an outlet.
 2. Aprocess as claimed in claim 1, wherein a plurality of reaction tubes (1)are connected in series and their mixing sections (14) are each assigneda main cold gas inlet line (17 a).
 3. A process as claimed in claim 1 or2, wherein the heat of reaction is removed by means of wall cooling (18)and the introduction of cold gas (17).
 4. A process as claimed in one ormore of claims 1 to 3, wherein the free-radical initiator is fed to aninjection region (11) by means of a carrier gas, cold gas main stream(17 a) or cold substream of the flowing medium which has been branchedoff before compression.
 5. A process as claimed in one or more of claims1 to 4, wherein the feed point (72, 81) for the free-radical initiatoris located downstream of the point where rotation is imparted to theflowing medium (61, 62).
 6. A process as claimed in one or more ofclaims 1 to 5, wherein the depth to which the free-radical initiator isinjected into the flowing medium (61, 62; 70) can be influenced by thegeometry of the outlet orifice (44) on the introduction finger (40). 7.A process as claimed in one or more of claims 1 to 6, wherein the feedfacilities for the flowing medium (61, 62) are at an angle (66) to oneanother of from 45 to 135°, but preferably 90°.
 8. A process as claimedin one or more of claims 1 to 7, wherein the flowing media (61, 62) eachpass through cross-sectional constrictions (63, 67) before they arecombined.
 9. A process as claimed in one or more of claims 1 to 8,wherein the rotating, flowing medium (61, 62, 70) passes through across-sectional constriction (71) downstream of an annular space (68)before reaching the feed point (72) for the free-radical initiator. 10.A process as claimed in one or more of claims 1 to 9, wherein thefree-radical initiator is fed into a shear gap (73) of the rotatingflowing medium (70) at the feed point (72).
 11. A process as claimed inone or more of claims 1 to 10, wherein the rotation in the flowingmedium (61, 62, 70) is generated by means of swirl elements (20, 80)located in the flow cross section (27, 28) upstream of the feed point(72, 80).
 12. An apparatus for preparing polyethylene in tube reactors,in which a free-radical initiator is fed into a flowing ethylene- andpossibly comonomer-containing medium (61, 62) and the flowing medium(61, 62) passes through a reaction tube (1) having a changing flow crosssection (27) and a free-radical initiator is introduced in a mixingregion (13), wherein substreams (61, 62) of the flowing medium impingeon one another at an angle (66) or swirl elements (20, 80) are locatedin the flow cross section (27, 28) and a feed finger (40) having anoff-center outlet orifice (44) for a free-radical initiator is locateddownstream of a constriction (71) in the rotating flow (70).
 13. Anapparatus as claimed in claim 12, wherein the outlet orifice (44) at thetip (41) of the feed finger (40) is inclined to the axis of the latterat an angle of from 5 to 80°, preferably 45°.
 14. An apparatus asclaimed in claim 12 or 13, wherein the swirl elements (20, 80) areprovided on their outer circumference with swirl blades (25, 82) whicheach extend over from 45 to 360°, preferably 90°, in the circumferentialdirection in an annular space (68) of the reaction tube (1).
 15. Anapparatus as claimed in claim 12 or 13, wherein the swirl elements (20,80) are provided on their outer circumference with swirl blades (25, 82)which each extend over from 45 to 360°, preferably 120°, in thecircumferential direction in an annular space (68) of the reaction tube(1).
 16. An apparatus as claimed in one or more of claims 12 to 15,wherein the diameter of the constriction (71) is from about 0.2 to 0.95times, preferably 0.7 times, the diameter D of the free flow crosssection (27, 28).
 17. An apparatus as claimed in one or more of claims12 to 16, wherein the free flow cross section (27) upstream of theconstriction (71) goes over at a total angle of from 10° to 70° into theconstriction (71).
 18. An apparatus as claimed in claim 17, wherein thetotal angle is particularly preferably 30°.
 19. An apparatus as claimedin one or more of claims 12 to 18, wherein the diameter 0.7×D of theconstriction (71) downstream of the feed point (71, 82) for thefree-radical initiator remains unchanged over a mixing section length(13) of from 10×D to 100×D.
 20. An apparatus as claimed in one or moreof claims 12 to 19, wherein the mixing section (13) goes over after from10×D to 100×D into the free flow cross section diameter D at a totalangle of less than 20°, preferably less than 14°.